The instant invention relates to an ignition control circuit and more particularly to a so-called electronic control circuit for automobile vehicles in which the spark of the ignition plug is obtained at the secondary of a voltage ignition coil when abruptly interrupting the current flowing through the primary of this coil.
Since a few years, it has been possible to use as a switch an electronic switch. FIG. 1 shows an embodiment of such a conventional electronic ignition system comprising an energy storage battery 1 (the vehicle battery) and an ignition coil 2, the secondary of which is connected with an ignition plug 3 and the primary of which is in series with an electronic switch 4, such as a Darlington circuit, allowing the current to flow or interrupting the current in the coil. A control device 5 permits to control, in relation with the information received from the engine 6, the switching on, then the switching off, of switch 4, this control device acts on the gate of the main switch which, in the given example, is of the bipolar-type. Therefore, when switch 4 is on, the current starts flowing through the primary of the coil and progressively increases. When switch 4 is off, a spark is produced in the ignition plugs 3.
In order to obtain a good operation of this device, at the opening of the power switch 4, the current attained in the primary of coil 2 has to be sufficient for causing the spark at the secondary. Ideally, the duration of the switching on of the power switch 4 should be provided so that this current value be precisely attained. In fact, since the engines operate at a variable speed, it is difficult to very precisely determine this duration and it is usually necessary to switch on the power switch 4 for a longer time duration than it is necessary. If no steps were taken, this would result in the flowing of a too important current through the primary of coil 2 and power switch 4.
In order to avoid those excess currents, current limitation means, an embodiment of which is shown in FIG. 1, are conventionally provided for.
Thus, a detection circuit 7 is serially connected with the power switch 4. This detection circuit comprises a resistor R1 having a low value in parallel with a dividing circuit comprising resistors R2 and R3, the voltage measured at the junction between resistors R2 and R3 being proportional to the current flowing through resistor R1. This measured voltage V.sub.D is compared with a reference voltage 8 in a controlled comparator-amplifier 9, the output of which supplies a base current to the power switch 4. Thus, at the switching on of power switch 4, when the current is low in resistor R1, the base current is maximum, and when the measured voltage is approaching the reference voltage 8, corresponding to the limit current, the base current is lowered. During this phase when the base current of the power switch 4 is lowered, the power switch no longer acts as a simple switch but as a current linear amplifier.
In this system, the whole set of components 4, 7, 8 and 9 forms a servo-control loop. In practice, this loop may prove to be unstable. The amplifier 9, implemented in an integrated circuit, is a high gain-amplifier, for example a three-stage amplifier. Owing to the present technologies, this gain may vary for each stage by a factor 3, for example between 100 and 200. Similarly, the gain of power switch 4, which is for example a multiple-stage Darlington circuit, may substantially vary, for example by a factor 30. Under those conditions, it is very difficult in practice to ensure a predetermined value to the gain of the servo-control loop and therefore the stability of the servo-control of the current.
Therefore, in practical systems, it is compulsory to strictly select the Darlington circuits, which restricts the choice of the suppliers and therefore increases the costs, and it is also necessary to use in the servo-control loop an external stabilization capacitor, which increases the number of terminals to be provided in the integrated circuit and also induces an increase in costs.